Add Swift (SwiftPrometheus) client lib

Author: MrLotU

content/docs/instrumenting/clientlibs.md

@@ -33,6 +33,7 @@ Unofficial third-party client libraries:
 * [Perl](https://metacpan.org/pod/Net::Prometheus)
 * [PHP](https://github.com/Jimdo/prometheus_client_php)
 * [Rust](https://github.com/pingcap/rust-prometheus)
+* [Swift](https://github.com/MrLotU/SwiftPrometheus)
 
 When Prometheus scrapes your instance's HTTP endpoint, the client library
 sends the current state of all tracked metrics to the server.